1. Field of the Invention
The present invention relates to a video signal processing apparatus which is adapted to a television receiving set or a video reproducer such as a video tape recorder (hereinunder referred to as "VTR") and a disk player so as to improve the signal-to-noise ratio of a video signal and the sharpness of a picture through noise reduction processing of the video signal.
2. Description of the Prior Art
The picture quality of a television receiving set or a video reproducer such as a VTR and a disk player is influenced by the balance among three factors, namely, the signal-to-noise-ratio of a video signal, the resolution and the sharpness.
FIG. 7 is a block diagram of the structure of a video signal processing apparatus in a conventional VTR which has been developed with a view to improving the signal-to-noise-ratio of a video signal and the sharpness. In FIG. 7, an FM video signal of 3.4 to 4.4 MHz picked up by a playback head enters a demodulator 1 to be demodulated to a video signal of the baseband. This video signal is supplied to a deemphasis circuit 2 which suppresses the high-frequency component of a video signal recorded while emphasizing the high-frequency component by a preemphasis circuit so as to reduce the high-frequency noise component penetrated on a recording and reproducing system of the video signal. That is, the deemphasis circuit 2 has the reverse characteristics of the preemphasis circuit.
The thus-deemphasized video signal is then supplied to a low pass filter (hereinunder referred to as "LPF") 3, which cuts off the high-frequency band of the video signal. Simultaneously with this, the deemphasized video signal is supplied to a quadratic differentiation circuit 4, wherein the transient portion and the noise component of the video signal are differentiated, and the level of the differentiated signal is attenuated to an appropriate level by an attenuator 5. The output signal of the LPF 3 and the output signal of the attenuator 5 are added by an adder 6. These LPF 3, quadratic differentiation circuit 4, attenuator 5 and adder 6 constitute a quadratic differentiation type picture quality improving circuit 23.
A high pass filter (hereinunder referred to as "HPF") 7 picks up the noise component included in the video signal from the output of the circuit 23. The picked up signal is then passed through a noise amplifier 8 for amplifying the noise component, a limiter 9 for limiting the amplitude of the noise component, a polarity inverter 10 and an attenuator 11, wherein the amplitude of the noise component limited by the limiter 9 is set at the optimum value.
The output signal of the quadratic differentiation type picture quality improving circuit 23 and the noise component signal with the polarity inverted are added by an adder 12. These HPF 7, noise amplifier 8, limiter 9, polarity inverter 10, attenuator 11 and adder 12 constitute a noise canceller circuit 24.
The operation of the prior art apparatus having the above-described structure will now be explained.
An FM video signal of 3.4 to 4.4 MHz, which has passed through the preemphasis circuit so as to be recorded while emphasizing the high-frequency component, is picked up by the playback head and input to the demodulator 1, thereby being demodulated to a video signal of the baseband. The demodulated video signal is then input to the deemphasis circuit 2 having the reverse characteristic of the preemphasis circuit so that the high-frequency component is suppressed so as to reduce the high-frequency noise component. Sufficient reduction of the high-frequency component, however, is not brought about solely by such a treatment of the emphasized high-frequency component, and a large noise component n remains, as indicated by (a) in FIG. 8. Especially, when the quality of a magnetic tape loaded on the VTR is poor, a considerable amount of noise n remains.
When the video signal, including a large noise component n as the signal (a) in FIG. 8, is supplied to the quadratic differentiation type picture quality processing circuit 23, the video signal first passes through the LPF 3 and the high-frequency component and the noise component are removed, thereby producing a signal indicated by (b) in FIG. 8.
On the other hand, the video signal also passes the quadratic differentiation circuit 4, wherein the transient portion t at which the level of the video signal is rapidly changed is differentiated two times and the noise component of the video signal is also differentiated. The level of the differentiated signal is attenuated to an appropriate level by the attenuator 5, thereby generating a waveform signal indicated by (c) in FIG. 8.
The signal output from the LPF 3 and indicated by (b) in FIG. 8 and the signal output from the attenuator 5 and indicated by (c) in FIG. 8 are added by the adder 6, thereby generating a signal indicated by (d) in FIG. 8. The signal (d) includes the noise component n but since appropriate peaks p1 and p2 are added to the rising transient portion of the original video signal (a) at which the video signal rapidly changes from the white level to the black level and the breaking transient portion at which the video signal rapidly changes from the black level to the white level, respectively, this video signal has increased sharpness in the picture quality.
The video signal (d), with the sharpness in the picture quality increased, is then input to the noise canceller circuit 24. The video signal input to the noise canceller circuit 24 passes through the HPF 7 and the noise amplifier 8, whereby the noise component n and the high-frequency component of the video signal are amplified, thereby producing a signal indicated by (e) in FIG. 8.
The amplitude of the noise component n is limited to L1 and L2 set by the limiter 9, thereby generating a signal indicated by (f) in FIG. 8. The polarity of the signal shown (f) is inverted by the polarity inverter 10. After the signal is attenuated by the attenuator 11 to an appropriate level, it is input to the adder 12 to be added to the signal (d), thereby generating a signal indicated by (g) in FIG. 8. The signal (g) is a processed video signal having a sharpness with the noise component n removed from the original video signal and provided with the peaks p1 and p2 at the rising transient portion and the breaking transient portion, respectively, by quadratic differentiation.
This prior video signal processing apparatus may be adapted as a video signal outline correcting means for emphasizing especially the outline portion of a video signal. The operation in this case will be explained as follows.
FIG. 9 shows the rising variation of a step waveform with the emphasized edge in the prior art which is described in "A Calculation Method of the Sharpness in a Television Picture" in THE TRANSACTIONS OF THE INSTITUTE OF ELECTRON-ICS, INFORMATION AND COMMUNICATION ENGINEERINGS JAPAN, Vol. J66-B No. 7 (1983, Jul.). As is clear from FIG. 9, the conventional correcting means for emphasizing the outline portion of a video signal obtains the quadratically differentiated signal of the waveform of the original signal, multiplies the signal by a certain coefficient, and adds the waveform of the original signal to the product.
With reference to FIG. 9, a waveform Q1 (X) is obtained by the quadratic differentiation of a step waveform I2 (=Q(X)), which is the original signal with the frequency band limited by a transmission pass, and the waveform Q1 is multiplied by a certain coefficient "-a" to obtain a waveform Id. The thus-obtained waveform Id is added to the original signal waveform I2 to obtain an outline correction signal waveform I2+Id.
If the rise time improvement with the outline correction means is represented by the time relative to the change in amplitude (response), it is V2/X2 for the outline corrected signal waveform I2+Id, while it is V1/X1 for the original signal waveform I2. It is thus clear that the rise time is considerably improved
The improvement of the outline portion of the video signal including an emphasis of the outline portion leads an improvement in the sharpness of the picture.
The above-described conventional method, however, is disadvantageous in that the noise in a video signal cannot be completely eliminated and the existence of the peaks p1, p2 for enhancing the sharpness of a picture cause new deterioration of the picture and emphasize the noise component.
More specifically, the peaks p1, p2 are formed fundamentally by quadratic differentiation in the prior art, as described above, and in order to provide the peaks p1, p2 with a certain degree of height by quadratic differentiation, the peak pulse width is also required to be enlarged considerably, as shown in FIG. 8, thereby producing the peaks p1, p2 with irregular shapes. These irregularly-shaped peaks exert deleterious influence on the subsequent noise reduction processing. Thus, the sharpness is greatly deteriorated by the formation of irregularly-shaped peaks.
In addition, when the peaks p1, p2 are produced in this way, ringing l is inevitably produced behind the peaks as in the signals (c), (d) and (g) in FIG. 8, thereby repeating periodic damping, which disadvantageously accelerates the deterioration of the picture quality.
Furthermore, since the output signal of the deemphasis circuit 2 is directly subjected to quadratic differentiation in the state in which the noise component of the video signal itself remains as it is, the noise component is enlarged in proportion to the peaks pl, p2 at the instant of the formation thereof, so that the improvement of the picture quality as a whole is not achieved.
When the conventional video signal processing apparatus is used for correcting video signal outline, the process consists of the steps of obtaining the quadratically differentiated signal of the waveform of the original signal, multiplying the signal by a certain coefficient, and adding the waveform of the original signal to the product. By this process, however, the outline correction signal obtained sometimes exceeds the level of the original waveform, as indicated by the hatched portion in FIG. 9, and when the original signal waveform has the maximum amplitude in the, dynamic range of the transmission system, the waveform is saturated.
If the quadratic differentiation is carried out so as to provide the peak values with a large amplitude, the pulse width of the peak is disadvantageously enlarged and the ringing l is generated, as in the signals (c),(d) and (g) shown in FIG. 8. Therefore, not only is the signal-to-noise ratio not improved as much as possible, but also the overshoot or preshoot is so strong as to display an unnatural picture or the ringing phenomenon causes an indistinct picture.